*5.2. Gradient Coatings*

Coating designs with gradient layers imply that coating properties change sequentially in the direction of the coating thickness. Frequently, the primary factor that is varied is the composition of the coating. Both the sputtering and arc deposition methods are used

to generate compositional gradients continuously or stepwise. Two examples of coatings with a Si gradient in which the layer containing less silicon is at the bottom and the layer with more silicon is at the top are shown to demonstrate this.

In the first example, CrAlSiN monolayers (0, 4, 1.8, 5.9, 8.5 at.% Si) and CrAlSiN coatings with a continuous Si gradient (Cr/Al ca. 1.6) were deposited by co-sputtering using elemental targets of Cr, Si, and Al with a Si content of 5 and 8 at.% in the top layer and were compared with (CrAlSi)N monolayers [122]. The authors described the formation of a nanocomposite (CrAlSi)N/a-SiN*<sup>x</sup>* structure for monolayer coatings with a Si content of 5.9, Cr23.4Alr15.6Si5.9N53.8O1.3, or higher by means of an X-ray diffraction pattern and for 8.5 at.% Si as well using TEM examinations. The gradient coatings exhibited higher toughness in terms of scratch/crack propagation resistance relative to monolayers with comparable Si content.

In the second example, (AlCrSi)N coatings were arc deposited using cathodes of Al70Cr30 and Al60Cr30Si10 as a gradient coating for the Si content [121,125]. Different levels of Si content were achieved by adjusting the number of cathodes per coating deposition step. The coatings containing Si showed a nanocomposite structure (AlCrSi)N/a-SiN*<sup>x</sup>* in all compositional regions. Cutting tests showed that the gradient coating provided better performance than the pure (Al70Cr30)N coating [121,125].

Coating designs with a gradient on the O content were realised by arc deposition in the top layer region of a part for solar energy thermal conversion in the form of (AlCr)N/(AlCr)NO/(AlCr)O. The functional coating properties indicate that this coating with its characteristic of selective absorption is a candidate for photo-thermal conversion at high temperatures [164].
